This invention relates to electro-optic displays. More specifically, this invention relates to reflective displays based upon electrically-addressable, color-selective media.
There have recently been developed displays that selectively reflect different colors in response to an applied electric field, and which do so not by relying on the use of conventional optical absorbers such as dyes or pigments but by exploiting optical effects such as interference and diffraction. Thus, for example, IMOD (interferometric modulator display) devices in which the spacing between a mirror and a partial reflector is controlled in response to an applied field may be constructed such that only certain wavelengths are strongly reflected, depending upon the relationship between the spacing and the wavelength of the light, as described for example in U.S. Pat. No. 5,835,255. More recently, synthetic photonic crystals have been developed whose refractive index varies periodically on a length scale comparable to the wavelength of visible light. Such materials behave similarly to minerals such as opals, in which Bragg diffraction causes certain wavelengths of light to be strongly reflected from the surface of the material while other wavelengths are transmitted through the structure. Which wavelengths are transmitted and which are reflected depends upon the lattice spacing of the photonic crystal. As described for example in U.S. Pat. Nos. 7,364,673, 7,616,376 and 7,826,131, photonic crystals may be made by colloidal self-assembly of for example silica particles, and such particles may be incorporated with a binder that fills the void space between the particles. If this binder material undergoes a dimensional change in response to, for example, an electrochemical reaction or other electro-activated process, the spacing between the silica particles is changed and the wavelength of light that is reflected is changed also. The wider the spacing between the particles, the longer the wavelength of light that is reflected. Devices have been built in which light from blue to near infra-red wavelengths may be selectively reflected. Such displays may hereinafter for convenience be referred to as “wavelength selective reflection” or “WSR” displays, and the medium used may be referred to as a “WSR medium”.
Light that is not reflected by the WSR medium is transmitted through the medium, and is typically absorbed by a dark surface provided on the opposed side of the medium from the viewing surface of the display (the surface through which an observer views the display). (It is not desirable that light which has passed through the WSR medium be reflected back through this medium since this will reduce the desired selective reflection brought about by the WSR medium, i.e., such reflection will tend to “muddy” the colors exhibited by the display.
Such WSR displays can typically achieve a good dark (black) state by moving the wavelength of maximum reflection into a range invisible to the human eye, such as the near-ultra-violet or the near-infra red, so that the WSR medium appears transparent to the human eye, which simply sees the dark backing surface. (All of the color changes in WSR displays are of course conducted on a pixel-by-pixel basis using techniques such as active matrix backplanes which will be familiar to those skilled in the technology of electro-optic displays.) Although WSR displays may be used to generate particular colors, corresponding to the optical band gap of the device, there has hitherto been no convenient way to reflect white light. The closest approximation to white pixels hitherto possible is a “process white” (somewhat analogous to the process black used in cyan-magenta-yellow printing systems), as shown in FIG. 1 of the accompanying drawings, in which there is a juxtaposition of red-reflecting, green-reflecting and blue-reflecting pixels on a black background. However, such a process white state only reflects one third of incident light, and therefore cannot produce a true white state.
Accordingly, there is a need for providing WSR displays with an improved white state, and this invention seeks to meet this need.